This invention relates to dropout compensation and, more particularly, to dropout compensation in an information signal that is recovered from successive tracks during a fast-speed playback mode when the playback transducer undergoes a track jump.
In a typical video recorder, video signals are recorded in successive, generally parallel oblique tracks on a magnetic tape. As is conventional, several horizontal lines of video information are recorded in each track; and in many conventional recorders a field interval is recorded in a track. During a playback operation at normal speed (i. e. at the same speed as was used for recording), a rotatable transducer scans an entire track to reproduce all of the horizontal line intervals recorded therein. The resultant video picture is of generally high quality with no noticeable distortion.
During a high speed playback operation, the transducer continues to rotate at the same constant speed used during a normal playback operation, but now the magnetic tape is driven at a higher speed. Consequently, the scanning trace of a transducer no longer is limited to a single track but, rather, the transducer now crosses a plurality of tracks to reproduce video signals from one track, and then from another, and another, and so on.
Timing errors that are expected to arise from the scanning of different tracks during a single scanning trace are corrected by the use of a time base corrector, known to those of ordinary skill in the art. In addition, dropouts which may be present in the video signal recovered from the magnetic tape, such as when a transducer passes over the guard band that normally is used to separate adjacent tracks, are compensated in accordance with known dropout compensation techniques. For example, when the signal level of the reproduced video signal drops below a threshold value, a level corresponding to a gray level is substituted for that low-level video signal. Hence, dropouts due to, for example, a track jump, are compensated by forcing the video signal to the gray level during each dropout interval. As a result, the video picture displayed therefrom includes gray-level images where the low-level video signal had been; and the display of a video picture having gray-level image portions inserted thereon is preferred to a video picture having dropout noise interference.
When the head jumps from one track to the next adjacent track, a portion of a horizontal line interval is recovered from the preceding track and the remainder of the line interval is recovered from the following track. If the horizontal line intervals in adjacent tracks are in precise alignment (known typically as H-alignment), the track jump of the transducer from one track to the next is expected to result in substantially no synchronization problem because the duration of the reproduced horizontal line interval pieced together from the preceding and following tracks is substantially equal to the horizontal line interval recorded in either track. Of course, that portion of the guard band scanned by the transducer produces dropout which is compensated in accordance with the aforementioned gray-level substitution.
However, in actual practice, the horizontal line intervals recorded in adjacent tracks often are not in precise H-alignment. The presence of tape jitter during the recording of one track but not during the recording of the next adjacent track may result in such incomplete H-alignment. Consequently, when a portion of a horizontal line interval is reproduced from one track and the remainder of the horizontal line interval is reproduced from the adjacent track, the overall length of the reproduced line interval may be greater than the standard length of a line interval recorded in either track. This enlarged line interval produced during a track jump is corrected by time base correction techniques; but the resultant video picture attributed to this correction of the reproduced line interval appears as cramped distortion. Furthermore, the phase of the chrominance signal reproduced from the track following the track jump is not synchronized to the horizontal synchronizing signal that is reproduced from the track which precedes the track jump, and this lack of phase synchronization is not easily corrected by typical time base correction techniques. This results in further distortion of the video picture that is reproduced during this high speed playback mode.
Therefore, the usual dropout compensation technique which substitutes a gray-level signal for the reproduced video signal when a dropout is detected, such as during the interval that the transducer scans the guard band, and the usual time base correction technique, which does not full satisfy a lack of phase synchronization in the chrominance signal, do not produce a video picture of satisfactory quality.